1. Field of the Invention
The present invention relates to an apparatus and method for measuring impedance, and more particularly, to an apparatus and method for measuring electrochemical impedance at high speed.
2. Description of the Related Art
An electrochemical impedance measuring apparatus measures current flowing at two electrodes when voltage is applied to the two electrodes placed in an electrolyte, to thus measure resistance and corrosion of electrodes of an electrochemical solution; an electrostatic capacitance due to the electric double layer between an electrode and an electrolyte; and reactivity at the electrode surface. Voltage obtained by adding a sine wave voltage having a predetermined frequency to a direct current (DC) voltage having the reaction potential of an electrolyte is used as signal voltage applied to the electrolyte. Current flowing when the signal voltage is applied to the electrolyte has a different phase than the voltage, and the amplitude of the current varies depending on the frequency. In this case, various useful information on the electrolyte can be obtained by computing the variation of the magnitude of an impedance Z, using a voltage V and a current I as in Equation (1) and computing the phase as in Equation (2) while varying the frequency of a sine wave voltage, and by presenting the results of computations on a complex number plane.                               "LeftBracketingBar"          Z          "RightBracketingBar"                =                              "LeftBracketingBar"            V            "RightBracketingBar"                                "LeftBracketingBar"            I            "RightBracketingBar"                                              (        1        )                                θ        =                              tan                          -              1                                ⁢                                    Z              IM                                      Z              RE                                                          (        2        )            
FIG. 1 is a diagram for explaining a conventional apparatus for measuring an electrochemical impedance. In FIG. 1, an adder 102 adds a sine wave voltage SIN(wt) generated by a sine wave generator 100 and a DC voltage E0 generated by a DC voltage generator 104. A constant voltage controller 106 applies a predetermined voltage (E0+xcex94Esin(wt)) to a reference electrode 112 and a working electrode 114 via a counter electrode 110. Current (I0+xcex94Isin(wt+xcex8)) flows between the counter electrode 110 and the working electrode 114 due to charge transferred through the electrolyte of an electrochemical cell 111. This current is introduced into a current-to-voltage converter 108 via the working electrode 114 and converted into a voltage. A real part correlator 118 receives a sine wave signal SIN(wt) generated by the sine wave generator 100 and an imaginary part correlator 116 receives a cosine wave signal COS(wt) generated by a phase shifter 120, which shifts the phase of the sine wave signal SIN(wt) by 90xc2x0 to generate a cosine wave signal COS(wt). The real part correlator 118 and the imaginary part correlator 116 convolve the sine wave signal SIN(wt) and cosine wave signal COS(wt) with a voltage applied to the electrolyte to compute the amplitudes and the phase difference of the real part and the imaginary part of the current. A computer 122 computes the impedance characteristic of the electrolyte using a program based on such data and displays the computed result on a monitor.
To measure the magnitude and phase difference of an impedance, the frequency of an applied voltage must be changed step by step typically from 0.001 Hz to 100 KHz while being convolved. To measure impedances for sine wave signals having different frequencies, since an impedance must be measured over at least one period at each frequency, it takes a very long time to measure, especially at a low frequency. In this case, when a voltage is applied to an electrode in an electrolyte for a long time, the concentration of the electrolyte at the diffusion layer between the electrolyte and the working electrode 114 changes from an initially measured concentration, so that it is difficult to perform a representative experiment. Moreover, the influences of charge transfer and mass transfer on the impedance of a solution cannot be effectively measured.
To solve the above problems, it is an object of the present invention to provide an apparatus and method for measuring an electrochemical impedance at high speed, in which the impedance of an electrolyte is measured at high speed, and the influence of charge transfer on the impedance and the influence of mass transfer on the impedance are separately measured.
Accordingly, to achieve the above object, the present invention provides a method for measuring the electrochemical impedance of an electrolyte at high speed. The method includes the steps of (a) applying a direct current (DC) voltage having the reaction potential value of the electrolyte to the electrolyte via a counter electrode and, after a predetermined time, applying a signal voltage, in which the DC voltage is added to a voltage of differentiated or integrated Dirac-delta function, to the electrolyte; (b) computing a digital data value related to only the voltage of differentiated or integrated Dirac-delta function among digital data, which is obtained by converting signal current flowing in a working electrode via the electrolyte into a voltage, integrating or differentiating the computed digital data value, and Fourier transforming the result of the integration or differentiation; and (c) obtaining changes in magnitude and phase according to frequencies based on the Fourier transformed value to compute the impedance.
The digital data value related to only the voltage of differentiated or integrated Dirac-delta function in the step (b) is the difference between a digital data value obtained after the reaction potential is applied to the electrolyte and a digital data value obtained after applying the signal voltage, in which the DC voltage is added to the voltage of differentiated or integrated Dirac-delta function, to the electrolyte.
The digital data value is differentiated and then Fourier transformed in the step (b) when the signal voltage, in which the voltage of integrated Dirac-delta function is added to the DC voltage, is applied to the counter electrode in the step (a), and the digital data value is integrated and then Fourier transformed in the step (b) when the signal voltage, in which the voltage of differentiated Dirac-delta function is added to the DC voltage, is applied to the counter electrode in the step (a).
The voltage of the integrated Dirac-delta function is 5-25 mV.
In another embodiment, the present invention provides a method for measuring an electrochemical impedance at high speed. The method includes the steps of (a) generating a direct current (DC) voltage having the reaction potential of an electrolyte, converting the DC voltage into a current, and applying the current to the electrolyte via a counter electrode; (b) after a predetermined time, converting a signal voltage, which is generated by adding a voltage of differentiated or integrated Dirac-delta function to the DC voltage, into a current and applying the current to the electrolyte via the counter electrode; (c) sampling an analog voltage which is applied to a working electrode via the electrolyte and converting the analog voltage into digital data; (d) computing a digital data value related to only the voltage of differentiated or integrated Dirac-delta function from the digital data, integrating or differentiating the digital data value, and Fourier transforming the integrated or differentiated result; and (e) obtaining changes in magnitude and phase according to frequencies from the Fourier transformed value to compute the impedance.
To achieve the above object, the present invention also provides an apparatus for measuring an electrochemical impedance at high speed. The apparatus includes a signal generator for generating and outputting a direct current (DC) voltage, which is the reaction potential of an electrolyte and outputting a signal voltage obtained by adding a step voltage to the DC voltage after a predetermined time, according to a control signal; a potentiostat for receiving the DC voltage and the signal voltage from the signal generator, applying the received voltage to a counter electrode of the electrolyte at high speed, and controlling a predetermined voltage to be applied to a working electrode using a reference electrode; a current- to-voltage converter for converting current flowing in through the working electrode into a voltage; a sampling unit for sampling the converted analog voltage and converting the analog voltage into digital data; and a central controller for controlling each element, storing the digital data obtained when the DC voltage is applied and when the signal voltage is applied at intervals, computing and differentiating the difference between the digital data, and obtaining changes in magnitude and phase at each frequency based on a value obtained by Fourier transforming the differentiated value so as to compute an impedance.
In another embodiment, the present invention provides an apparatus for measuring an electrochemical impedance at high speed. The apparatus includes a signal generator for generating and outputting a direct current (DC) voltage, which is the reaction potential of an electrolyte and outputting a signal voltage obtained by adding a step voltage to the DC voltage after a predetermined time, according to a control signal; a galvanostat for converting the DC voltage and the signal voltage into a constant current and applying the constant current to a counter electrode of the electrolyte; a sampling unit for sampling an analog voltage applied to the working electrode through the electrolyte and converting the analog voltage into digital data; and a central controller for controlling each element, storing the digital data obtained when the DC voltage is applied and when the signal voltage is applied at intervals, computing and differentiating the difference between the digital data, and obtaining changes in magnitude and phase at each frequency based on a value obtained by Fourier transforming the differentiated value so as to compute an impedance.